In a typical fiber optic gyroscope, there are polarization phenomena that cause erroneous rotation detection by the gyroscope. Some polarization errors are caused by light being cross-coupled from one polarization state to another. In gyroscopes having, for example, a proton exchange integrated optical circuit, cross-coupling occurs at the coupling of integrated optical circuit with a light source and an optical fiber sensing loop. A phase modulator on such integrated optical circuit affects one polarization state of light differently than another for a given signal applied to the modulator. Certain origins and characteristics of various polarization errors appear to have not been known in the art prior to the present invention. Discoveries of such origins and characteristics and ensuing solutions are presented here.